1. to push a body part or and inanimate object(most often a sex toy) with force into the male urethra. A very uncommon way of masturbation and/or pleasure.
Made famous by the internet video Kids in a sandbox

A software that manage how others programs contained inside the sandbox can interact with the outside, establishing limits to the main environment by giving a false sense of freedom to trapped softwares. Sandboxie is an example of a sandbox software.

When someone calls you a sandbox they are referring to the term "son of a beach", and the son of a beach is probably a sandbox. So, they are pretty much calling you a little bitch. Tell it to your friends.